The present invention is related to fuel cells. In particular, the present invention is related to fuel cells including metal hydride electrodes.
A fuel cell is an electrochemical device in which the chemical energy of a conventional fuel is converted directly and efficiently into low voltage electrical energy. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and remote power supply applications.
Fuel cells, like conventional batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel (preferably hydrogen), and the oxidant (preferably either oxygen or air that contains oxygen) are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells also offer a number of important advantages over engine or generator systems. They include relatively highly efficient, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation.
A schematic diagram of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in FIG. 1. Referring to FIG. 1, the major components of a typical fuel cell 10 is an anode 14 for hydrogen oxidation, a cathode 16 for oxygen reduction and an electrolyte layer 12. In the embodiment shown, the anode 14 and the cathode 16 are each in contact with and positioned on opposite sides of the electrolyte layer. During operation, a continuous flow of fuel, commonly hydrogen, is fed to the anode 14 while, simultaneously, a continuous flow of oxidant, commonly oxygen or air, is fed to the cathode 16. In the example shown, the hydrogen is fed to the anode 14 via a hydrogen compartment 13. Likewise, the oxygen or air is fed to the cathode 16 via an oxygen/air compartment 17. As noted in FIG. 1, one side of the anode 14 is in contact with the electrolyte layer 12 while the other side is in contact with the hydrogen compartment 13. Likewise, one side of the cathode 16 is in contact with the electrolyte layer 12 while the other side is in contact with the oxygen/air compartment 17.
The hydrogen fuel is oxidized at the anode with a release of electrons through the agency of a catalyst. These electrons are conducted from the anode 14 through wires external to the cell, through the load 18, to the cathode 16 where the oxidant is reduced and the electrons are consumed, again through the agency of a catalyst. The constant flow of electrons from the anode 14 to the cathode 16 constitutes an electrical current that can be made to do useful work. Typically, the reactants such as hydrogen and oxygen, are respectively fed through the porous anode 14 and cathode 16 and brought into surface contact with the electrolyte 12. The particular materials utilized for the anode 14 and cathode 16 are important since they must act as efficient catalysts for the reactions to take place. In certain types of fuel cells, such as alkaline fuel cells, the catalytic material used for the anode may comprise a hydrogen storage alloy material (also referred to as a metal hydride material). Hence, the anode may be a hydrogen storage alloy electrode (also referred to as a metal hydride electrode). Examples of such alkaline fuel cells that use hydrogen storage alloy materials are provided in U.S. patent application Ser. No. 09/524,116, the disclosure of which is incorporated by reference herein.
One of the crucial steps in the preparation of a hydrogen storage alloy electrode for use in a fuel cell is that of xe2x80x9cactivationxe2x80x9d. Activation increases the catalytic properties of the hydrogen storage alloy.
Activation, it is believed, increases the surface area and alters the chemical composition and/or structure of the hydrogen storage alloy bulk and/or the hydrogen storage alloy surface. The activation process enhances the properties of the hydrogen storage alloy material for operation as a catalytic material for the fuel cell anode.
Activation is believed to result from 1) removal of reducible surface oxides which tend to interfere with the functioning of the material, 2) reduction of particle size resulting from an increase in volume, which fractures the alloy particles, and 3) changes in the chemical composition and/or structure of the alloy and/or the surface of the alloy.
It is noted that good catalytic properties, high rate capability and large surface area are important factors for a hydrogen storage alloy electrode when used as an anode of a fuel cell. The hydrogen storage alloy material serves as a catalyst for the dissociation of hydrogen gas fuel into hydrogen atoms. Storage capacity and high loading of the hydrogen storage alloy material, while important for many battery applications, are secondary for fuel cell applications.
Activation of a hydrogen storage alloy battery electrode may be achieved through the surface treatment of the electrode by subjecting the electrode to an alkaline or acidic etching treatment. This type of surface treatment alters the surface oxides of the hydrogen storage alloy. An example of a hot alkaline etch treatment is provided in U.S. Pat. No. 4,716,088, the disclosure of which is incorporated by reference herein. However, for fuel cell applications, this form of activation may not be feasible. The present invention is directed to an alternate activation process which uses electrical current to activate the hydrogen storage alloy electrode used in a fuel cell.
Disclosed herein is a method of activating a hydrogen storage alloy electrode, comprising the step of:
applying a plurality of current cycles to the electrode, each of the current cycles including a forward current effective to at least partially charge the electrode and a reverse current effective to at least partially discharge the electrode.